Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received August 24, 2016; final manuscript received September 6, 2017; published online October 19, 2017. Assoc. Editor: Mhamed Boutaous.

Abstract

Entrained air in oil can cause malfunctions and damages within hydraulic systems. In this paper, we extend existing approaches to reduce the amount of entrained air by separating air bubbles from oil using filter elements. The aim of this study was to investigate the ability of different untreated and surface modified woven and nonwoven fabrics (NWF) to separate air bubbles from oil when directly integrated into an intake socket of an oil pump. An experimental setup was constructed to generate entrained air in oil and to characterize changes in oil aeration and pressure drop induced by the filters. Measurements were conducted at volume flow rates of 2.2 and 5.4 l/min with an inflow angle normal to the filter elements. The developed setup and aeration measurement method proved to be suitable to generate entrained air in oil in a reproducible manner and to accurately characterize aerated oil up to air contents of about 5%. Significant influences on the aeration characteristics were found only for the NWF. Whereas the number of air bubbles decreased by up to 33% relative to the values in the oil reservoir for a flow rate of 2.2 l/min, a significant reduction of the volumetric air ratio could not be achieved as resulting bubble distributions comprised a higher number of large bubbles. We suggest that the lack of effective bubble separation was a result of the flow-induced pressure drop by the filters, which increased with the flow rate.

Performed image processing steps with ImageJ to determine the rate of oil aeration for each oil sample. (a) Original image after cropping and scaling (image size: 1600 × 1166 pixels/approximately 2.0 × 1.5 mm). (b) Image after conversion to black and white using auto thresholding. Incomplete and truncated air bubbles at the image edges are colored gray. (c) Final processed image after elimination of incomplete and truncated bubbles (gray areas in (b)). The smallest complete air bubble in (c) has a diameter of 22 μm and the largest of 244 μm.

Influence of pump back pressure and air supply on the oil aeration. (a) Measured relative bubble area in dependence of the measured back pressure of the circulation pump. (b) Calculated relative bubble volume plotted over measured back pressure. Relative bubble area and bubble volume determined from USB microscope images with a size of: 2000 × 2000 pixels/approximately 8.5 × 8.5 mm.

Aeration and deaeration process for a pump back pressure of 2 bar, an oil flow rate of approximately 6.5 l/min and an air supply of 100 ml/min. Development of mean bubble count (a), mean relative bubble area (b), and mean relative bubble volume over time (c) (n = 5, confidence interval (CI) of mean 95%). The shaded regions indicate the deaeration process with the circulation pump shut off; the dotted lines mark the mean value calculated from the time interval 5–10 min. Development of air bubble diameter distribution over time during the aeration process (d).

Return to: Separation of Entrained Air Bubbles From Oil in the Intake Socket of a Pump Using Oleophilic and Oleophobic Woven and Nonwoven Fabrics

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